In performing various operations such as welding, coating, cleaning, energy beam irradiation, machining, etc., using an industrial robot, it is known that a taught track given as an on-line or off-line teaching is corrected using detected signals from a sensor which is supported on the robot, and the robot is controlled so that the point of operation of a tool supported thereon traces a desired track. Available as a sensor for track correction is, for example, a laser sensor which detects the positions of lines of operation by deflecting a laser beam. The lines of operation include feature lines, such as weld lines formed on the workpieces (e.g., a borderline defined by the difference in level between two workpieces to be welded) and outlines of the workpieces, which can be identified by the sensor.
Such a control method for a robot with a sensor is highly effective in the case where an intended operation can be carried out by only moving the tool while slightly modifying the taught track as a reference. In practice, however, it is difficult to use this control method with position correction in such a case where a desired track must be achieved by accurately executing necessary position correction while aiming at a track which is greatly deviated from the taught track, as in the case of so-called weaving welding.
The reason is that the control method with position correction, in which a track extending substantially parallel to the line of operation is slightly modified, cannot directly be applied to the position correction of weaving tracks. Since the weaving tracks are deviated in the direction transverse to the line of operation, the tool moves at a considerably high relative speed. Thus, in such a condition, it is hardly possible to obtain accurate data for position correction when an attempt is made to detect the operation line by the sensor and to correct the robot position based on the detection, so that accurate position correction can not be achieved.
Since the period of output of the sensor is limited (e.g., about 0.1 second), it is very difficult to specify the moment that the operation line is detected by the sensor (or the moment that the line of operation is crossed by a laser beam from the visual sensor), and to provide correction data accurately in response to current position data to the robot at the specified moment, on a real-time basis.
Because of such a reason, there is no successful precedent that the control method for correcting the position of a robot using the signals detected by the sensor has been effectively applied to the operation such as the weaving welding.
In many welding operations, a technique of the so-called overlap welding is used in order to obtain a durable welding by moving a welding torch to follow the same track along the weld line once traced or along a track slightly shifted from a previous trace. Conventionally, in such a case, the processing of moving the welding torch with the positional correction using the sensor is simply repeated. Thus, position correction data obtained in a first cycle are not effectively utilized for a second and subsequent cycles of overlap welding and subsequent cycles. Besides, the condition in the vicinity of weld lines is changed due to the effect of the first cycle of welding, and so in some cases, sensing in the second and subsequent cycles is not effectively carried out.